Camera view is broken
We experienced the same issue here, though in our case the Viewport Render would not work either any more. However, we found out that it is linked to the Blender Screen Area, not the camera. We noticed this because we actually had more than one 3D View in our layout, and only one of them would show the described behavior. We checked again and again, and all properties were set correctly. They were indeed identical across the areas.

To fix it, switch another screen area to a 3D View. The camera should behave as it is supposed to there. Now collapse the broken one. You can also switch to a different screen layout using the Dropdown box in the header: